Towards Zeptosecond-scale Pulses from X-ray Free-electron Lasers

The aim of the workshop was to consider what developments might be achievable in the next generation of light sources. One area of particular interest is the generation of ultra-short pulses of light. High harmonic generation (HHG) sources are now capable of reaching pulse durations in the range of tens to hundreds of attoseconds, thereby facilitating the study of electron dynamics. Here we consider the potential of free-electron lasers (FELs) to contribute to this field. First we make the case for FELs in general as a particularly promising source of ultra-short pulses, and summarise some of the relevant proposals. Different classes of ultra-short pulse techniques are identified—each with respective merits and potentially better suited to different types of application. Particular focus is given to highlighting a recent proposal by the authors to generate trains of few-cycle pulses from amplifier FELs, which if applied at hard x-ray wavelengths could generate pulses at the single attosecond scale, or even shorter—into the zeptosecond scale.